VIENNA — Intense negotiations with Iran have yielded "tangible progress," U.S. Secretary of State John Kerry said Tuesday, but significant gaps remain ahead of a July 20 target date for a deal meant to put firm curbs on Tehran's nuclear activities in exchange for an end to sanctions.

Kerry, who joined the foreign ministers of Britain, France and Germany on the weekend to add diplomatic muscle to the talks, said the negotiations would continue until at least Sunday. In the meantime, he said, he would consult with President Barack Obama and the U.S. Congress on where the talks are if no pact is agreed on by July 20.

Both sides may face a hard-sell on extension. Many Congress members are already skeptical about the sense of trying to get a negotiated reduction in Iran's nuclear program and oppose going on with them. Any decision to go past July 20 would also be likely criticized by hard-liners in Iran, who fear the talks will result in a scaling-back of their country's nuclear program.

"While there is a path forward, Iran needs to choose to take it," said Kerry, adding he believed the two sides can determine "the precise contours of that path."

He spoke after multiple conversations with Iranian Foreign Minister Mohammed Javad Zarif and separate meetings with the Western foreign ministers.

An extension of up to six months is possible for the negotiations, by mutual agreement. But judging by Kerry's comments, any decision to go past Sunday will be taken in the closing hours leading up to that date.

The main dispute is over Iran's nuclear enrichment program. Tehran says it needs to expand enrichment to make reactor fuel but the U.S. fears Tehran could steer the activity toward manufacturing the core of nuclear missiles and could use its present capacity to produce enough weapons-grade uranium for one warhead in only several months. The U.S. wants deep enrichment cut for at least 20 years; Iran wants to greatly expand enrichment over less than a decade.

Kerry did not detail the substance of his talks with Zarif. But a diplomat familiar with the conversations said they focused not only on enrichment but on other disputes, including how to diminish proliferation dangers from a reactor that would produce substantial amounts of plutonium when completed. Like enriched uranium, plutonium can be used to make nuclear weapons, and the diplomat said the Iranians, are ready to reconfigure plans so that less plutonium is generated.

But their model could be reverse-engineered and the Americans want a totally converted model that cannot be reconverted to increased plutonium output. The diplomat demanded anonymity because he is not allowed to discuss the confidential negotiations.

For the U.S. and Iran, an extension makes sense in multiple ways. With so much at stake, and the potential for U.S.-Iranian cooperation in other areas being explored, neither side wants to terminate the nuclear talks. Such a course could put Tehran back on a path toward isolation and even set the stage for U.S. military action if Iran proceeded again toward nuclear weapons capacity.

Page 2 of 2 - But they must take into account the difficult internal pressures against a deal — or an extension for that matter.

David Albright, whose Institute for Science and International Security is often consulted by the U.S. government on Iran's nuclear program said that a full six-month extension of the talks makes sense only "if Iran accepts that the number of its centrifuges must be reduced and the Arak reactor converted permanently."

"If the six cannot get at least two major concessions now, they are unlikely to get them and the others needed six months from now," he said in an email.